The invention relates to a motion-measuring system of a machine, a machine comprising a motion-measuring system, a method for operating a motion-measuring system of a machine, and a method for operating a machine comprising a motion-measuring system.
Machines, such as e.g. coordinate measuring machines or machine tools, usually have a movable part, which e.g. carries a sensor for capturing coordinates of a workpiece or carries a processing tool for processing a workpiece. The movable part is therefore a sensor carrier, in particular. The movable part is movable within a movement range relative to another part of the machine, e.g. relative to a base.
By way of example, the sensor of the coordinate measuring machine (for short: CMM) is a measuring head mounted on the movable part (for example a sleeve or an arm) of the CMM. On the measuring head it is possible to mount a probe (e.g. a probe pin), in particular, using which the CMM probes the surface of the workpiece in a tactile manner in order to generate the sensor signals of the measuring head. Therefore, in particular, a probe for the tactile probing of the workpiece to be measured is also an example of a sensor or of a part of the sensor.
The measuring head has a sensor system, in particular, which generates measurement signals whose evaluation enables the coordinates to be determined. However, other sensors also crop up in coordinate measuring technology. By way of example, the sensor may merely initiate the measurement of the coordinates. This is the case for example for a switching measuring head which generates a switching signal upon contact with the workpiece to be measured, which switching signal initiates the measurement of the coordinates e.g. by reading off the scales of the movable part or parts of the CMM. In principle, the sensors can be classified into sensors that carry out measurement by contact (tactile probing of the workpiece) and sensors that do not carry out measurement by contact. By way of example, optical or capacitive sensors for coordinate measurement are sensors which are not based on the principle of tactile probing. Furthermore, it is known to use invasive radiation, penetrating into the interior of the measurement object, for coordinate measurement. Moreover, it is possible to classify sensors according to the type or size of the in particular simultaneously detected region of the workpiece. In particular, sensors may measure coordinates just of a point or of an area on the surface or else in the interior of the workpiece or measure coordinates of a volume of the workpiece. By means of computed tomography, for example, a three-dimensional image of the measurement object can be created from measurement results of radiation detectors. In addition, it is possible to use different sensors simultaneously on the same sensor carrier or on different sensor carriers, either as separate units or integrated into a common unit. The different sensors can employ identical and/or different measurement principles.
It is customary to configure a CMM such that the sensor can be exchanged for a different sensor. In this case, that part of the CMM which has the interface for mounting the respective sensor can be referred to as a sensor carrier. However, that part of the coupled sensor which is immobile relative to the coupling interface in the coupled state can also be referred to as part of the sensor carrier. Moreover, as e.g. in the already mentioned case of a measuring head with a tactile probe mounted thereon, it is possible for two different parts to be designated in each case as a sensor. If one sensor carries the other sensor, said one sensor can be referred to as a sensor carrier of the other sensor.
The sensor serves for capturing coordinates of a workpiece. Signals generated by the sensor from a scan of the workpiece are not sufficient by themselves, however, to be able to determine the coordinates of the workpiece in the coordinate system of the coordinate measuring machine. Information about the position and/or alignment of the sensor is additionally required for this purpose. The CMM therefore has a position determining device for ascertaining a position and/or an alignment of the sensor carrier carrying the sensor and thus of the sensor. Additional motion-measuring devices are usually combined with the movable parts of the CMM. By way of example, a material measure, for example a scale graduation with lines, is arranged on one part of the CMM and a measurement signal transducer is arranged on a second part of the CMM, said second part being movable relative to said first part.
An evaluation device of the CMM determines coordinates of the workpiece from a position and/or alignment of the sensor carrier as ascertained by the position determining device and from signals of the sensor. When the term position determination is used in this description, it should be understood alternatively or additionally to mean a determination of an alignment of the respective part or region, unless different substantive matter is evident from the context.
The position determination of the sensor carrier depends on external influences and the respective operating state of the CMM. By way of example, the temperature and air humidity influence the position determination. Furthermore, the weight force of a sensor coupled to the sensor carrier must be taken into account. Moreover, parts of the CMM may flex depending on the position of the sensor carrier. The speed of the movement of the movable parts of the CMM and the acceleration likewise influence the position measurement. CMMs are therefore calibrated for specific ambient conditions and operating states. Care must then be taken to ensure that the ranges of the influencing variables that are taken into account in the calibration are not left during operation of the CMM. The complexity for the calibration and the corresponding correction models is high on account of the multiplicity of influencing variables. Furthermore, the uncertainty that remains, despite the correction, in the measurement variables measured by the CMM differs in magnitude in different subranges of the influencing variables. Moreover, the behavior of the CMM changes over the course of time, with the result that the calibration must be repeated.
With the exception of the differences between the coordinate measurement and the workpiece processing, the statements about the position determination analogously also apply to machine tools having a tool carrier which is movable in a movement range and which carries or can carry a processing tool. Such machine tools, too, usually have a position determining device.
The calibration of the position determining device can be at least simplified if an optical position determination takes place by camera images being recorded by at least one camera. By evaluating the camera images, given suitable configuration and arrangement of the camera or cameras, it is possible to determine the position of the movable part with high accuracy. As an alternative or in addition to the position determination with at least one camera, it is possible to use a position determining device with a non-imaging optical sensor system, for example with a correlation sensor.
As already mentioned, other motion-measuring devices are also customary, however, particularly in coordinate measuring machines. Their measurement results are used in particular for controlling the movement e.g. in accordance with a predefined movement sequence. Alongside the motion-measuring devices with a scale graduation as already mentioned, tachometers are usually used, too, which measure the movement of motors of the machine, in particular directly. The tachometer signals can be used directly by the motor controller. However, they can e.g. also be transmitted as redundant measurement results of the movement to a superordinate controller of the machine.
High-precision motion-measuring devices such as are required as part of coordinate measuring machines and machine tools are comparatively complex in their production and must be calibrated.
It is an object of the present invention to specify a motion-measuring system of a machine, a machine comprising a motion-measuring system, a method for operating a motion-measuring system of a machine and/or a method for operating a machine comprising a motion-measuring system in which the complexity for production and calibration can be reduced.